The increasing need for hydrocarbons, particularly from regions of political stability, has led to reevalution of the desirability of recovery of residual oil left in formations previously worked. Thus, oil fields having had a first production run using conventional production techniques, having later had a secondary production using more sophisticated production techniques, are now being considered for third generation or tertiary recovery methods. These methods are naturally more expensive than the primary and secondary methods mentioned. Accordingly, it is desirable to determine the amount of residual oil left in a given field as precisely as possible so that an accurate determination of whether or not it would be economically feasible to perform tertiary recovery techniques can be made. Furthermore, the amount of residual oil is also relevant to selection of a particular tertiary oil recovery method.
In 1978, the Interstate Oil Compact Commission (IOCC) of Oklahoma City, Okla., published a volume entitled "Determination of Residual Oil Saturation" in which were tabulated the various methods of determination of residual oil saturation. The Commission concluded at page 289 that, "Even if all necessary precautions are observed in the design and conduct of tests to determine residual oil saturation, it should be recognized that there are large areal variations in oil saturations remaining in reservoirs that are substantially depleted by water flood or water drive".
Accordingly, there remains a need in the art for an accurate method of determining residual oil saturation. In order to be practicable the method must be capable of performance at reasonable cost. Furthermore, such a method would desirably measure residual oil saturation in regions beyond the immediate vicinity of the borehole. By comparison, many of the methods considered by the IOCC were only capable of determining residual oil saturation within 12 to 18 inches of the borehole wall. Another disadvantage of several of the methods considered by the IOCC were that the methods were only applicable when the hole was uncased, i.e., had not had a steel casing inserted therein. Many useful wells have such casings and it is therefore desirable that any method of determining residual oil saturation be operable in such cased wells.